Kelly Legault, Ph.D., P.E. USACE SAJ
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Regional Sediment Management Approach 2 RSM Operating Principles Recognize sediment as a regional resource connect beaches & inlets Evaluate use of all sediment sources & sinks Optimize operational efficiencies & natural exchange of sediments Balanced, economically viable, environmentally sustainable solutions Improve economic performance by linking multiple interacting projects Consider regional impacts Adaptively manage
Why Nearshore Berms? Sand is a Resource: Sand is a valuable resource. Every effort should be made to retain it to the nearshore. Nearshore Berms may provide - a supplementary route in addition to beach nourishment. New numerical modeling methods and field data - including CMS and LIDAR provide new tools for design analysis OFFSHORE 0% d outer = the closure depth of the shoal/buffer zone d inner = the closure depth of the littoral zone fn=? 100%
In the context of Regional Programs and Scales, what are the Engineering Considerations for Nearshore Berm Design? Tides Waves Currents Sediment Types
Engineering with Nature: Given what we do know about the character of the nearshore, how do we optimize placement to retain sediment to the littoral zone? Tides Waves Currents Sediment Types
In a nutshell...... Sediment Transport is: That s pretty easy.... It s just the where and when that s the tough part.
Sediment Transport is Described by the Advection Diffusion Equation DIFFUSION ADVECTION Bed Stress Currents Waves Sweeps the sediment up into the water column VELOCITIES Gravity Sediment Size/ Shape Sediment Density Brings sediment to the bed SEDIMENT Currents Upwelling/ Downwelling Wind Surf Zone Currents Waves FLOW
What do I need to know? Turbulence Wave orbital velocities Current velocities Broken Waves WAVES CURRENTS Sediment characteristics SEDIMENT Surf zone currents Broken Waves Undertow Alongshore Currents SURF ZONE HYDRODYNAMICS
Nearshore Waves Waves are important because they provide: Turbulence Pick Up Sediments at the Bed Wave Refraction Longshore Currents Wave Breaking Cross-Shore Currents
Deep Water Waves No Pick-Up OFFSHORE ONSHORE crest L = Length d =depth bottom--y I. WAVES and WAVE CHARACTERISTICS
Wave Shoaling OFFSHORE ONSHORE... as waves move onshore: Friction @ BED Wave Shape
Wave Refraction OFFSHORE ONSHORE... as waves move onshore: Alongshore Currents
Shallow Water Waves Pick Up OFFSHORE ONSHORE
Wave Breaking OFFSHORE ONSHORE Wave Breaking = Turbulence and Transport Breakers pick up large amounts of sediment and transport landward
Wave Run-Up - Undertow OFFSHORE From the Breakers ONSHORE Shoreline interrupts flow returns as undertow
Putting it all together on the Beach Turbulence Wave orbital velocities Current velocities Broken Waves Gravity Sediment characteristics Currents Surf zone currents Broken Waves Undertow Alongshore Currents How do these forces influence beach profile evolution?
Destructive Forces that move sediment offshore... an incomplete list Gravity. Tries to make a profile horizontal High Turbulence. Picks up sediment in the surf zone and, with the help of gravity, moves them offshore Undertow. Seaward return of wave-induced mass transport (surface roller)
Constructive Forces that move sediment onshore SHALLOW WATER WAVES... an incomplete list Strong Velocity under the Crest sweeps coarse grains shoreward. Weak velocities under Trough keep fines in suspension and they move offshore in the longer duration trough. A means of natural sorting.
Constructive Forces that move sediment offshore... an incomplete list Run-Up. Shoreward transport (last stage of wave dissipation) Wave Bore. Shoreward wave-induced mass transport (surface roller)
Mimicking Nature: Destructive Forces and migration of sand bars Winter Waves
Mimicking Nature: Constructive Forces and migration of sand bars Small waves pitch forward on the Bar
Engineering with Nature: Create a nearshore berm that behaves similarly to a summer bar Shallow Waves. Hallermeier Depth of Closure 1981,1983: d inner (H 2, T 2 ), d outer (H,T,d 50 ) d outer waves and sediment characteristics d inner wave climate, alone fn= WAVES, sediment 0% 100%
Engineering with Nature: Large Scale Alongshore Advection and Diffusion. d outer d inner
Engineering with Nature: Optimizing Placement. 0% d outer d inner 100% Cross shore Alongshore
Why Nearshore Berms? Sand is a Resource: Sand is a valuable resource. We find a good surrogate in nature, nearshore bars, about which we have a good understanding. Nearshore Berms may provide - a supplementary route in addition to beach nourishment which will retain sediment to the littoral zone New numerical modeling methods and field data - including CMS and LIDAR provide new tools for design analysis. CSHORE and GENESIS coming online as well. Given the wealth of existing knowledge on cross-shore transport and predictive tools, we are in an excellent position to consider nearshore berms as an additional option for sediment placement. 0% 100%